S. Rusiecki

Low temperature anomalies and pressure effects on the structure and Tc of the superconductor YBa2Cu4O8 (Tc=80 K)

Abstract The structure of the 80 K superconductor YBa 2 Cu 4 O 8 has been measured as a function of temperature and pressure between 5 K and 165 K by high resolution neutron powder diffraction. The oxygen bridging CuO planes to double CuO chains moves away from the chains, toward the planes as T c increases with increasing pressure, indicating charge transfer to the planes. A similar effect occurs when YBa 2 Cu 3 O 6+ x is oxidised to YBa 2 Cu 3 O 7 , again increasing T c . The a - and c -axes of YBa 2 Cu 4 O 8 increase smoothly with temperature, but the b -axis, parallel to the direction of the CuO chains, appears to have a minimum near T c . This results in a slight inflexion in the orthorhombic distortion near T c , similar to that observed in YBa 2 Cu 4 O 7 . The oxygens within these chains appear disordered even at low temperature, again as in YBa 2 Cu 4 O 7 . However, in YBa 2 Cu 4 O 8 oxygens from the CuO-planes also have relatively large out-of-plane amplitudes. High resolution electron microscopy and diffraction show that YBa 2 Cu 4 O 8 , unlike YBa 2 Cu 3 O 7 , does not have planar structural defects; there are no twins or superstructures.

Phase diagrams of YBa2Cu4O8 and YBa2Cu3.5O7.5 in the pressure range 1 bar≤PO2≤3000 bar

Abstract The P-T-x phase diagram of the pseudobinary system (Y-Ba-Cu-O)-O 2 has been further investigated in the oxygen pressure range between 1 and 3000 bar. The stability ranges of the phases YBa 2 Cu 4 O 8 (124), YBa 2 Cu 3.5 O 7.5− x (123.5) and YBa 2 Cu 3 O 7− x (123) have been determined. Long duration experiments showed that the 123 phase is not stable at least down to 7 bar≤ P ≤20 bar oxygen and 900°C. It is not clear whether at lower pressures and temperatures the 123 phase is thermodynamically stable or metastable due to low reaction rates. In the presence of excess CuO, the 124 is the stable phase. The melting of 124 pellets at P O 2 =2800 bar shows that even at this pressure the 124 compound melts incongruently. Using the phase diagram data we could change the T c of 123.5 from 16 to 70 K by varying systematically the nonstoichiometry. Due to a narrow homogeneity range the T c of 124 remained constant but is different for powder pellets (81 K) and for crystals (70 K), probably due to the influence of the flux. Single crystals of both 124 and 123.5 with dimensions up to 4 mm were grown from the flux under high oxygen pressure.

Copper NQR and NMR in the superconductor YBa2Cu4O8+x

Abstract The frequencies νQ, the linewidth and the spin-lattice and spin-spin relaxation times T1 and T2, respectively, of the 63Cu nuclear quadrupole resonance (NQR) signals for chain and planar copper sites in YBa2Cu4O8 were measured between 6 and 750 K; in addition nuclear magnetic resonance (NMR) spectra were taken at 150 and 300 K. The results are compared with data known for the related structure YBa2Cu3O7. Above Tc the tensors of the electric field gradient and the anisotropic Knight shift at the copper sites differ in a subtle way from those of YBa2Cu3O7. At the Cu2 site, the derivative of νQ with respect to temperature changes its sign around 200 K. No indication for magnetic ordering at the Cu sites was found. Below Tc there is no evidence for BCS type relaxation and the Cu2 Tc rates vary like exp( T T 0 ) and the Cu1 rates like Tn with n≈1.35. Above Tc the Cu1 rates suggest the presence of quasiparticles (and phonon interactions) while the Cu2 rates can perhaps be explained by a model of Monien et al. postulating the coupling of Cu nuclei to itinerant interacting quasiparticles.

The nonstoichiometry of the high-Tc superconductor Y2Ba4Cu7O15±x (14 K⩽Tc⩽68 K)

Abstract In this paper we present a study of the thermodynamic, structural and physical properties of Y2Ba4Cu7O15±x, the new high-Tc superconductor we have discovered recently. The nonstoichiometry of Y2Ba4Cu7O15±x has been varied between -0.70 T c , 1 2 =26.5 K ) indicating that possibly a coupling over long distance (11.5 A) takes place.

High resolution neutron powder diffraction investigation of temperature and pressure effects on the structure of the high-Tc superconductor Y2Ba4Cu7O15

Abstract We have investigated the effect of temperature and 10 kbar pressure on the structure of the new high-temperature superconductor Y 2 Ba 4 Cu 7 O 15 ( T c =68 K) in an attempt to relate the pressure dependence of T c in such materials to changes in certain Cu-O distances. The orthorhombic Ammm X-ray model of Bordet et al., with lattice constant c ≈50.3 A, has been confirmed by our high-resolution neutron powder diffraction measurements. The structure can be described as alternate layers of single CuO-chain (YBa 2 Cu 3 O 7 ) and double CuO-chain (YBa 2 Cu 4 O 8 ) blocks. The thermal expansion is anisotropic, with the b -axis, parallel to the single and double Cu-O chains, remaining approximately constant for T T c . As in YBa 2 Cu 4 O 8 , Cu-O distances bridging chains and planes, as well as Ba-O distancesare significantly changed by pressure. However, in Y 2 Ba 4 Cu 7 O 15 these changes are different for the single and double CuO-chain layers. A correlation with the pressure dependence of T c is proposed.

The high Tc superconducting phases of the Y2Ba4Cu6 + nO14 + n family

Abstract A brief overview is presented of some thermodynamic, structural and physical properties of the HTc compounds YBa2Cu3O7 (single chains; nonstoichiometric; Tc = 92 − 55 K), YBa2Cu4O8 (double chains; stoichiometric; Tc = 80 K) and Y2Ba4Cu7O15 (alternating single and double chains; nonstoichiometric; Tc = 70 − 14 K). This family of compounds is an excellent model system for the study of the influence of the chains in superconductivity. A great advantage of YBa2Cu4O8 is the high oxygen stability (up to 850 °C) and the narrow nonstoichiometric range, both due to the stability of the double chains. The oxygen stability keeps orthorhombicity and hinders a structural phase transition like that in YBa2Cu3O7. Therefore, single crystals of YBa2Cu4O8 (and Y2Ba4Cu7O15) do not have a high concentration of twins. Crystals without twins are easily grown and allow for the first time the investigation of the a-b anisotropy which is shown to be appreciable. Although we have put great efforts in the investigation of the thermodynamic phase diagram of the Y-Ba-Cu-O system, the slow kinetics do not allow yet an exact determination of the phase boundaries. The up to now existing data are discussed and the dependence of the Cu Ba ratio on the oxygen pressure is presented.

Evidence for Ca substituting Ba in YBa2−xCaxCu4O8±y: a nuclear quadrupole resonance investigation

Abstract For the understanding of the T c enhancement in Ca doped YBa 2 Cu 4 O 8± y it is crucial to know which crystallographic site the Ca +2 ion is occupying. We investigated this problem by measuring the Cu nuclear quadrupole resonance (NQR) spectrum at 100 K in YBa 2 Cu 4 O 8± y doped with 10% Ca ( T c = 91 K ). The NQR lines of the plane and chain Cu sites appear at the same frequencies v q as in the pure compound, however, both lines are broadend by static defects. Within the linewidths no temperature dependence of v q was observed and no indication for magnetic ordering at the Cu sites was found. At 20 600 Mhz, near to the Cu(1) line, a resonance line has been detected which is absent in the pure YBa 2− x Ca x Cu 4 O 8± y spectrum. Based on NQR arguments we assign this new line to Cu(1) nuclei whose electric field gradients are disturbed by Ca +2 ions occupying Ba +2 sites. We concluded that a major fraction of Ca ions occupy Ba sites, therefore the increase of T c by Ca doping cannot be explained by an increase of the hole concentration.

Spin susceptibility and spin dynamics at Cu sites in Y Ba2Cu4O8

Abstract New measurements of the Knight shift tensor K at both Cu sites in the normal conducting state of the 82 K superconductor 1-2-4 are reported. The data are in accordance with a picture where at both Cu sites the Cu valence is close to 2+ and the hole on the plane site is quasi-localized. A significant temperature dependence at plane Cu site is found which is associated with temperature dependent antiferromagnetic fluctuations. Knight shift and spin-lattice relaxation times at Cu plane site are interpreted to be dominated by antiferromagnetic correlations with a temperature dependent correlation length.

Measurement of the London penetration depths in YBa2Cu3Ox by means of muon spin rotation (μSR) experiments

Abstract To get accurate values of the London penetration depths in YBa2Cu3O x , μSR measurements were performed on a high quality, sintered sample and a c-axis-oriented polycrystal. For the sintered sample the temperature dependence of the effective penetration depth λeff is well described by the two-fluid model, with λeff(0) = 155(10) nm. This behavior of λeff(T) is consistent with conventional s-wave pairing. The anisotropy ratio λ c λ ab ⋍ 5(1) was determined from measurements on the polycrystal. These results were used to calculate λab(0) = 130(10) nm and λ c (0) ⋍ 500 – 800 nm .

17O NMR study in aligned YBa2Cu4O8 powder

We present 17 O nuclear magnetic resonance (NMR) experiments in YBa 2 Cu 4 O 8 . Using an uniaxially oriented powder we were able to distinguish the four inequivalent oxygen sites in YBa 2 Cu 4 O 8 . The electric field gradient and the magnetic shift tensors have been determined at 100 K for all oxygen sites. Good agreement has been found between the experimental electric field gradient tensor and the one obtained from linearized augmented plane wave calculations. We further report on the temperature dependence of the in-plane and out-of-plane magnetic shift of the plane oxygens in the temperature range of 85–350 K. The results are compared with related data from YBa 2 Cu 3 O 7 and YBa 2 Cu 3 O 6.63 .